In data systems especially for use in computer systems, the data is recorded on a media in the form of ones and zeros, i.e., in digital form, which in magnetic recording are indicated on the media itself by the magnetic orientation of certain areas thereof. For using the data within a computer system, there must be synchronized therewith a clocking pulse which times the various functions of the system with the data. In very early day systems such clocking pulses were recorded directly on the media with the data being interspersed therebetween. However in present day systems, the clocking pulse is provided by an oscillator within the data recovery system which oscillator must be adjusted both in frequency and phase to match that of the data being read.
The adjustment of such oscillators is complicated in present systems by the fact that the oscillator must function at a harmonic frequency greater than the data frequencies because presently used codes allow for the nonrecording of pulses at predetermined locations to indicate data in the same manner as recorded pulses. The the clocking signal generator, however, must continue to function even though no data pulses are present for a predetermined period of time.
There is provided on the recording media, along with the recorded data, an initial series of pulses known as a preamble which is sensed for synchronizing the frequency and phase of a signal from the clock signal generator with that of the preamble. With the codes used today that include missing bits as a routine part of the code, it is necessary also that the clock signal generator be able to accommodate for missing bits. This dictates that the clock signal generator must be of a harmonic type for actual data recovery that is, it must operate at a frequency which is a harmonic or multiple of the data frequencies. However, in attempting to set or lock-in the clock during the preamble stage, it has been found that frequently such a harmonic clock will lock on an incorrect frequency. Naturally if the clock does this, difficulties arise when subsequently reading the data.
It is also necessary that the clock signal generator be capable of locking onto the preamble signal quickly so that a minimum space on the recording medium is used for the preamble. One solution used in the past has been to broaden the band width of the harmonic clocking pulse generator so as to increase the capability of the generator to lock rapidly onto the preamble signal. However the increase in band width increases the possibility the clock will lock onto an improper frequency. One solution to improper locking-in of the signal generator is to provide a clocking pulse generator of the nonharmonic type. Such a generator not only would adjust in phase to the preamble data, but also would only lock in on the exact frequency of that data. However as pointed out before, a harmonic type clock signal is required for data recovery when the data is recorded with certain bits deliberately omitted as in present day codes. Therefore, the usual answer has been to utilize a harmonic clocking signal generator with a narrow band width during the preamble locking-in phase to lessen the possibility of the generator locking onto improper frequency, but this requires a longer preamble.
It is the object of this invention to provide a clocking signal generator which takes advantage of the benefits of both harmonic and nonharmonic type generators by operating in two modes, that is, it operates in the nonharmonic mode during the locking-in period and in the harmonic mode during the readback operation.